Insulated spool for electromagnets



Nov. 21, 1939. 1.. o. LARSEN 2,130,420

INSULATED SPOOL FOR ELECTROMAGNETS Filed Oct. 31, 1936 lA/l/[NTOR L 0.ZARSEN A 77'0RNEY Patented Nov. 21, 1939 UNITED STATES INSULATED SPOOLFOB ELECTRO- MAGNETS Leonard 0. Larsen, Downers Grove, Ill., assignor toWestern Electric Company, Incorporated,

New York, N. 1., a corporation of New York Application October 31, 1936,Serial No. 108,684

BClalms.

This invention relates to insulation, and more particularly to insulatedcoil spools and a method of producing insulated spools.

Certain types of electromagnetic coils are 6 wound on preformed spoolshaving cores of ma netic material and complete and efiective insulationis required between the spool and the wire wound thereon as well asbetween successive windings of the coil. Because of their superiordielectric properties and stability, certain derivatives of cellulose,such as cellulose acetate, are

particularly suitable for this purpose. When insulating a spool withthese materials, which are most conveniently applied in sheet form, itis 15 difficult to provide adequate insulation at the juncture betweenthe core and the spool heads. In some cases this joint has been sealedwith a plastic, such as pyroxylin cement, which is-costly to apply andnot entirely satisfactory from a M service standpoint because any excesscement interferes with the subsequently applied winding and lack ofuniformity in the seal may result in service failure of the coil.

Objects of this invention are to improve the insulation of spools forelectromagnetic coils and to provide economical and eilicient methodsfor their manufacture.

In accordance with one embodiment of the invention, insulation isprovided for a coil spool of conventional construction having a core ofmagnetic material and a pair of heads of insulating material securedthereto. A sheet of cellulose acetate is wrapped around the core and asplit washer of cellulose acetate is mounted 35 against the inner faceof each spool head, said washer having a slightly concave surfaceabutting the spool head, a central aperture encircling the core, and aperipheral ferrule extending therefrom over the insulation on the core.Windings placed on the core are insulated with a sheet of celluloseacetate inserted between the windings and having corrugated edgeportions compressed against the spool heads.

A complete understanding of the invention may 45 be had from thefollowing detailed description taken in conjunction with the appendeddrawing in which:

Fig. l is a side elevation, partly in section, of a coil spool providedwith insulation in accord- 50 ance with one embodiment of the invention;

Fig. 2 is a view, partly in section, of a completed coil wound on thespool shown in Fig. 1;

Fig. 3 is a perspective view of an insulating washer shown in Fig. 1;

Fig. 4 is a perspective view of a sheet of insulating material embodyingan alternative form of the invention, and

Fig. 5 is an enlarged partial section of a coil insulated with thecorrugated sheet shown in 00 Fig. 4.

Referring now to the drawing, the invention is particularly adapted to aspool Ill of the type used extensively for relays and other electricalapparatus. The spool has a core ll, usually a soft magnetic metal oralloy, and a spool head I! or flange, generally made of fibre or othernonconducting material, mounted on knurled end portions I! at each endof the core or otherwise rigidly secured thereto.

To insulate the metal core effectively from the subsequently appliedwinding, the section of the core between the heads is covered with asheet ll of cellulose acetate or other material having satisfactorydielectric properties. Thin sheet material is used for this purpose andpreferably applied in' a plurality of layers to develop maximumbreakdown strength. In one type of coil employed in communicationequipment relays three layers and a lap of .002" thick cellulose acetatesheet were used on the core with satisfactory results.

After the sheet material is wrapped around the core it is coalesced withacetone,- other suitable solvent or heat, leaving the end portionsimmediately adjacent to the heads unsealed. A srlit disk or washer l5made of cellulose acetate and having a slightly concave or dished bodyis placed around the core and abutted against the inner side of eachspool head with the concave surface of the washer against the head. Thewasher has a central aperture i6 (Fig. 3) conforming with the coreinsulation contour and an integral ferrule or flange lI extending fromthe aperture towards the center of the spool over the core insulation.

In mounting the washer, it is spread at the split portion is, placedaround the core and slid along the core against the head. When thewasher is compressed against the head its periphery initially engagesthe head and this frictional contact forces the ferrule against theunsealed end portion of the cellulose acetate sheet on the core. Thesheet material flares out and is compacted under the ferrule to providea smooth, well insulated support for the end helices of the bottom layerof the winding l9. Tension due to the concavity of the washer holds itsperiphery tightly against the spool head and prevents the wire frompassing between the washer and the head during the winding operation. Ina sense the ferrule serves as a sleeve to hold the unsealed ends of theinsulating strip M in place and the body portion of the washer is anintegral flange on the sleeve to insure an impervious layer ofinsulation at the juncture of the strip and the head. Since the washerextends over the whole inner face of the head, the latter may be made ofa material having strength as its dominating property and less attentionneed be paid to its insulating properties.

Aim the insulating sheet and washers can be made of various ceilulosederivatives or other insulating materials, such as paper or fabric.celluloseacetateispreferredbecauseofitssuparlor electricalcharacteristics, suitability for forming and adaptability for sealing.

Theinsulatingwashersareformed fromcellulose acetate sheet rapidly andeconomically in heated dix shaped to the proper contour. M'aterialremoved to provide the central aperture is formed or extruded into theflange thus avoiding wasteandthe concave shapeisimpressedinthe washersduring the forming operation. The dimensions of the washers aredetermined by the sine and service requirements of the coil in whichtheyareused. Forcertaintypesof relaycoils washers formed from .010"thick cellulose acetate sheet are satisfactory, but widely varyingthicknesses can be used for other applications. The degree of concavityrequired in the washer body willalsovarybutforsmalllightwashersveryslight dishing is adequate.

In certain coils, aplurality of windings are applied to the spool andcomplete insulation is required between adjacent layers of thesuccessive windings. Sheet insulating material has been employed forthis purpose, but with this method the end helices of the wire arediilicult to insulate adequately because the edges of the sheet did notconform to the surface of the wire adjacent to the heads. To overcomethis difliculty an insulator sheet II of cellulose acetateisformedasshowninFigQ. Aflatbodyil of the sheet conforms approximatelyto the length of the winding and terminates in a series of continuouscorrugations 22 formed along each end of the sheet and extending beyondthe body portion. As the end layer of the wire is applied over thesheet, the surplus material in the corrugations nests around the wireagainst the head, as shown in Fig. 5, to adequately insulate thisportion of the coil.

In the preparation of the interleaving sheet the corrugations arereadily impressed in the sheet by means of a heated die or by runningthe sheet through a pair of heated meshed gears or suitably shaped milsengaging the edge portions of the sheet.

An insulator of this description can also be used on the cores inconjunction with the formed washers and is also suitable for insulatinglead out wires, resistance wires or other areas within the coil or forother purposes. The increased length of material at the edges of thestrip due to the corrugations lessens very markedly the danger ofstresses occurring which would be of suflicient force to start a tear inthe sheet.

Other modifications and adaptations of the specific embodimentsdisclosed herein are feasible, and it is to be imderstood that theinvention is limited only by the scope of the appended claims.

What is claimed is:

LAmethodofinsulatingaspoolhavinga core-and spaced heads on the corecomprising the steps of wrapping a sheet of cellulose acetate around thecore portion between the heads, coalescingthecellulose acetatesheetatthecenter of the core, and anchoring each lmsealed end portion of thecellulose acetate sheet with a split concave washer of cellulose acetatematerial mmmtedasainsttheinnermrfaceofthespooi head. nhlwalrer havingapaipherylyinginaplaneandadaptedtolieagahuttheinnerawwasheradiaeenttotheperiflrerydishedawayfromthephneoftheperlphery.

zametbodofhmlatingaspodhavinga coreandaheadsecm'edtheretopreparatorytocave surface of the washer against the spool head,andmovingthewasherintocloseengagementwiththeheadsaidwasherhavingaperipherylyinginaplanelsflnsttheinnerfaceof thespool head and the adjacent portion ofthewasherbeingdishedawayfromtheplane of the periphery to compress theperiphery of thewashercloselyagainsttheheadoutofthe path of thesubsequently applied wire.

3.Amethodofinsulatinga spool havingacoreandspacedheadssecuredtothecorepreparatory to winding wire on thespool, comprising mounting a split apertured circular disk of celluloseacetate material against the inner surface of each head with theapertured portion of the disk engaging the core, the disk beingpreformed into a concave shape having a periphery lying in a plane andadapted to lie against the inner surface of the spool head with theremaining portion of the disk being'dished away from the plane of theperiphery for holding the disk in close engagement with the head andpreventing the subsequently applied wire from passing between the diskand the head.

4. An insulating washer for a coil spool having a core and a headsecured to the core, comprising a split body with a central apertin-efor encircling the core, the periphery of said body lyinginaplaneandbeingadaptedtolie against the inner face of said head and theportion of said body jacent to the periphery being dished away from theplane of the periphery, and a flange extending from the body at theapertm'e for engaging the core.

Y 5. In an electromagnetic device, a spool having a head and a core, andan insulating washer adjacent to the inner face of the head. comprisinga peripheral portion lying in a plane and engaging the inner face ofsaid spool head, a central portion engaging the core, and anintermediate portion extending from the periphery of the washer to thecore-engaging portion dished away from the plane of the periphery into anormally concave shape which is distorted when the washer is positionedagainst the head for urging the peripheral and central portions of thewasher respectively into clou contact with the head and core.

6. In an electromagnetic device, a coil spool having a metal core andheads secured thereto. a strip of insulating material wrapped around thecore, and a split concave washer of cellulose acetate material forinsulating the spool at the juncture between the core and the head, saidwasher having a periphery lying in a plane against theinner face ofthespoolheadwiththe remaining portion of the washer being dished awayfrom the plane of the periphery for the purpose of preventing wire thatis subsequently applied to the spool from passing between the washer andthe head.

LEONARD 0. LARBEN.

